1. Field of the Invention
The invention relates to a separator for a solid polymer electrolyte fuel cell using a solid polyelectrolyte and a process for producing the same. This type of separator is sometimes called a xe2x80x9cbipolar platexe2x80x9d in the art. For convenience, the term xe2x80x9cseparatorxe2x80x9d will be used throughout the entire specification.
2. Prior Art
Conventional fuel cells using a solid polyelectrolyte have a construction, for example, as shown in FIG. 1A, i.e., a construction comprising; a solid polyelectrolyte film 50; and two electrode films (an anode-side electrode film 51 and a cathode-side electrode film 52) sandwiching the solid polyelectrolyte film 50 therebetween. In each of the electrode films, a catalyst layer, such as a platinum layer, is provided on and integrally with the solid polyelectrolyte film 50 side. Numeral 57 designates a sealing material for sealing the solid polyelectrolyte film 50 and the periphery of each of the anode-side electrode film 51 and the cathode-side electrode film 52 disposed respectively on both sides of the solid polyelectrolyte film 50. Numeral 54 designates an anode-side separator which is abutted against the anode-side electrode film 51. Grooves 53 for anode gas, such as hydrogen gas, are provided between the anode-side separator 54 and the anode-side electrode film 51. Numeral 56 designates a cathode-side separator which is abutted against the cathode-side electrode film 52. Grooves 55 for cathode gas, such as oxygen gas, are provided between the cathode-side separator 56 and the cathode-side electrode film 52. Both the separators 54, 56 should be formed of a material which is impermeable to gas and is electrically conductive. In general, the separators are fabricated of a carbon plate.
The mechanism of a reaction in the above fuel battery cell 58 will be explained. In the anode-side electrode film 51, hydrogen gas, which has been externally supplied through the grooves 53 for anode gas, is passed through a gas diffusion layer within the electrode film 51, reaches near a reaction zone, and is absorbed into the catalyst to form active hydrogen atoms. As shown in the following formula, the hydrogen atoms are reacted with hydroxyl ions in the electrolyte to give water. In this case, two electrons are passed through the electrode film 51 and are transmitted to the other electrode side through an external circuit.
H2+2OHxe2x88x92xe2x86x922H2O+2exe2x88x92
On the other hand, in the presence of the catalyst, the cathode-side electrode film 52 receives two electrons from the electrode film 51 side, and oxygen molecules, which have been externally supplied through the grooves 55 for cathode gas, are reacted with water from the electrolyte to produce hydroxyl ions.
xc2xdO2+H2Oxe2x86x922OHxe2x88x92
The hydroxyl ions produced in the cathode-side electrode film 52 move through the electrolyte and reach the anode-side electrode film 51, and an electrical circuit is formed as a whole.
Therefore, the reaction in the whole fuel cell is as follows.
H2+xc2xdO2xe2x86x922H2O
That is, in the reaction, hydrogen in the fuel gas is reacted with oxygen in the air to produce water.
In actual fuel cells, a fuel cell stack 60 having a laminate structure as shown in FIG. 2 is adopted. The fuel cell stack 60 has a structure comprising: a laminate of a large number of fuel battery cells in a plate form as shown in FIG. 1A; and pipings for supplying fuel gas (hydrogen gas and oxygen gas) to the laminate. Regarding the pipings, in FIG. 2, numeral 61 designates an anode gas introduction pipe, numeral 62 an anode gas discharge pipe, numeral 63 a cathode gas introduction pipe, and numeral 64 a cathode gas discharge pipe.
In the fuel cell stack 60, the separators used generally have a structure suitable for lamination, as shown in FIG. 1, wherein an anode-side separator 54 and a cathode-side separator 56 have been integrally provided respectively on the front and back sides. A specific structure thereof is as shown in FIGS. 3 and 4. FIG. 4 is a cross-sectional view taken on line Axe2x80x94A of FIG. 3, FIG. 3A a left side view of FIG. 4, and FIG. 3B a right side view of FIG. 4.
Regarding this separator, in FIG. 1, the grooves 53 for anode gas in the anode-side separator 54 and the grooves 55 for cathode gas in the cathode-side separator 56 are grooves which each extend in a direction perpendicular to the paper surface and are parallel to one another. In FIGS. 3 and 4, the grooves 53 for anode gas are grooves extending in the vertical direction of the paper surface of FIG. 3A, while the grooves 55 for cathode gas are grooves extending in the left and right direction of the paper surface of FIG. 3B. The grooves 53 for anode gas cross the grooves 55 for cathode gas in the front-and-back-side relationship.
Regarding this separator, as shown in FIGS. 3 and 4, in the anode-side separator 54, both ends of the grooves 53 for anode gas are respectively in communication with an introduction passage hole 65 connected to the anode gas introduction pipe 61 and a discharge passage hole 66 connected to the anode gas discharge pipe 62. On the other hand, in the cathode-side separator 56, both ends of the grooves 55 for cathode gas are respectively in communication with an introduction passage hole 67 connected to the cathode gas introduction pipe 63 and a discharge passage hole 68 connected to the cathode discharge pipe 64.
The separators 54, 56 having the above structure are generally prepared by forming grooves in a carbon plate. Therefore, for strength reasons, there is a restriction on an increase in opening area of the grooves 53 for anode gas and the grooves 55 for cathode gas. This poses a problem that the pressure loss is large in each gas passage and the supply efficiency of the fuel gas is lowered by a level corresponding to the pressure loss.
Further, in general, the carbon plate has excellent electrical conductivity and contact resistance. The carbon plate, however, is mechanically brittle. This poses a problem that, in forming the grooves, the width of peaks provided between grooves should be made wide to some extent for avoiding a lack of mechanical strength.
Further, since the peak portions are used in the state of being strongly pushed against the electrode films, in this pushed portion, the fuel gas (hydrogen gas) cannot be supplied to the electrode film side. This lowers the supply efficiency of the fuel gas and thus disadvantageously deteriorates power generation efficiency by a level corresponding to the supply efficiency lowering level.
Furthermore, in this type of fuel cell, the use of pure oxygen as an oxidizing agent is not cost effective, and there is a demand for the use of air per se. Since, however, the concentration of oxygen in the air is as low as about 20%, when the discharge of the air is not successfully carried out, the oxygen concentration on the cathode-side electrode film 52 side is relatively lowered in relationship with the fuel gas (hydrogen gas), disadvantageously leading to deteriorated power generation efficiency.
Further, on the cathode-side electrode film 52 side, protons are reacted with oxygen to give water, and, consequently, there is a fear of this water clogging the gas passage.
When all the above facts are taken into consideration, the fuel battery cell should inevitably has a very complicate structure.
On the other hand, a proposal has been made on a technique wherein a material is used which has been produced by providing a metal, which can realize complicate working with high accuracy, as the material for the separator and covering the surface of the metal with a metal nitride as a protective layer (Japanese Patent Laid-Open No. 353531/2000 entitled xe2x80x9cSEPARATOR FOR SOLID POLYMER ELECTROLYTE FUEL CELL AND PROCESS FOR PRODUCING THE SAMExe2x80x9d).
This technique is advantageous in that, when the metal is used as a separator, a thin separator can be fabricated with high accuracy. Since, however, the protective film is a metal nitride, the electrical conductivity and the anticorrosive property are not fully satisfactory. Further, a burden is disadvantageously imposed on the formation of the protective layer.
Further, a proposal has been made on a technique wherein a material is used which has a structure produced by providing an electrically conductive and corrosion-resistant stainless steel plate as a metallic separator, subjecting this stainless steel plate to forming, providing a tin plating as a first coating on the formed product, for improving antioxidation properties and electrical conductivity of the stainless steel plate, and covering the tin coating with graphite as a second coating, for maintaining the corrosion resistance (Japanese Patent Laid-Open No. 138067/2000 entitled xe2x80x9cGAS SEPARATOR FOR FUEL CELL, FUEL CELL USING SAID GAS SEPARATOR FOR FUEL CELL, AND PROCESS FOR PRODUCING SAID GAS SEPARATOR FOR FUEL CELLxe2x80x9d).
In this technique, the tin plating as the first coating functions as an antioxidation film for a base metal, and the graphite as the second coating functions as a layer for inhibiting the corrosion of the antioxidation film (when the antioxiation film is a plating, the surface has a structure having micropores and thus is likely to be attacked) (anticorrosive function).
In this technique, an example is described wherein, beside tin plating, a nickel, titanium, or conductive ceramic coating is used as the first coating. When nickel or titanium is used, the specific resistance of the oxide thereof is very high and is close to that of the insulating material or semiconductor. Therefore, when oxidation proceed in a wide area, the electrical conductivity as the separator cannot be maintained (i.e., the surface resistivity is increased) even though the oxidation of the base metal can be prevented. In this connection, the following fact should be noted. The graphite as the second coating per se is electrically conductive, is chemically stable, and is anticorrosive and, in addition, poses no problem of ion elution, but on the other hand, is mechanically brittle, is likely to cause cracks or pinholes with the elapse of time. For this reason, it is difficult to say that the graphite has excellent anticorrosion effect. Due to this nature, during use for a long period of time, although the graphite per se is not deteriorated, the first coating is corroded. This corrosion functions as an origin of corrosion, and the base metal also is attacked and the corrosion region is widened. As a result, ions of the constituent metal is eluted. This poses a problem that the catalyst and the electrolyte are deteriorated and the characteristics as the fuel cell is disadvantageously deteriorated. Further, in this connection, it should be noted that, since the tin plating as the first coating has a structure such that micropores are present on its surface, the corrosion resistance is unsatisfactory and this is also considered causative of the widening of the corrosion region.
Here the resistance of C (sintered graphite) is 10+3 xcexcxcexa9.cm2, whereas the resistance of tin oxide, nickel oxide, and titanium oxide is 10+4 xcexcxcexa9.cm2 for SnO2, 10+6 xcexcxcexa9.cm2 for NiO, and 10+7 xcexcxcexa9.cm2 for TiO2. That is, as compared with tin, the resistance of nickel oxide and titanium oxide is incomparably larger than that of tin. Incidentally, the resistance of the metals is 12.8 xcexcxcexa9.cm2 for tin and 55 xcexcxcexa9.cm2 for titanium.
The surface resistivity refers to resistance as measured by a method wherein both surfaces of a plate sample having a predetermined dimension is pressed by a press under a predetermined pressure and, in this state, the resistance between both the surfaces of the sample in the direction of thickness is measured as the surface resistivity. The surface resistivity somewhat varies depending upon the pressure applied by the press, the thickness of the sample, and the area of the plate sample. When these values are substantially equal for each sample, the surface resistivity value varies according to the state of oxide formed on the surface of titanium or titanium alloy. The pressure is applied by the press for making the contact between the surface of the electrode plate and the surface of the plate sample satisfactory over the whole surface. When the pressure applied by the press is proper, there would be little or no influence of the thickness of the plate samples and the difference in area between samples on the surface resistivity.
Accordingly, it is an object of the invention to solve the above problems of the prior art and to provide a separator for a solid polymer electrolyte fuel cell which has good workability and, at the same time, can surely maintain or improve electrical conductivity and corrosion resistance and to provide a process for producing the same.
According to the first feature of the invention, a separator for a solid polymer electrolyte fuel cell, comprises: a cladding material formed by covering the surface of highly conductive copper, aluminum, magnesium, iron, or alloy thereof with highly anticorrosive titanium or titanium alloy through plastic working by rolling or extrusion; and a carbon material covering at least one surface of the cladding material.
In the separator according to the first feature of the invention, the surface of the titanium or titanium alloy in the cladding material has an oxide film having a thickness in the order of nm and the cladding material has a surface resistivity of not more than 5 mxcexa9.cm2.
In the separator according to the first feature of the invention, 5 to 20% of the whole thickness of the cladding material is accounted for by the covering of the titanium or titanium alloy in the cladding material and the thickness of the cladding material is 0.1 to 2 mm.
In the separator according to the first feature of the invention, 5 to 20% of the whole thickness of the cladding material is accounted for by the covering of the titanium or titanium alloy in the cladding material and the thickness of the cladding material is 0.1 to 0.5 mm.
According to the second feature of the invention, a process for producing a separator for a solid polymer electrolyte fuel cell comprises the steps of: covering the surface of highly conductive copper, aluminum, magnesium, iron, or alloy thereof with highly anticorrosive titanium or a titanium alloy by plastic working to form a cladding material, wherein, after the covering of the surface of the highly conductive copper, aluminum, magnesium, iron, or alloy thereof with copper, the plastic working is carried out and the copper present on the surface of the cladding material is then removed; and then covering at least one surface of the cladding material with a carbon material.
According to the first and second features of the invention, the use of a cladding material formed by covering the surface of inexpensive and highly iron, conductive copper, aluminum, magnesium, or alloy thereof with highly corrosion-resistant titanium or titanium alloy through plastic working by rolling or extrusion permits a film of titanium or a titanium alloy, which per se has excellent corrosion resistance, to be formed as a dense film which has no micropores on its surface and has better corrosion resistance. Further, the covering of the surface of the film with a carbon material can significantly inhibit the oxidation of the titanium or titanium alloy underlying the carbon material, in addition to the anticorrosion effect of the material per se. As a result, the electrical conductivity and corrosion resistance as the separator for a fuel cell can be surely improved. According to the invention, by virtue of a combination of the effect attained by the cladding material of titanium or titanium alloy particularly produced by plastic working and the effect of the covering of the carbon material, a special effect can be attained such that the oxidation of the surface of titanium or titanium alloy can be significantly inhibited and, in particular, the oxide formation region can be limited to a local area.
Further, in the invention, for the covering of the titanium or the titanium alloy in the cladding material, the adoption of a specific construction, wherein the thickness of the oxide film on the surface of the cladding material is in the order of nm and the surface resistivity of the whole cladding material is not more than 5 mxcexa9xc2x7cm2, permits the electrical conductivity as the separator for a fuel cell to be maintained in a suitable range for a long period of time. When the thickness of the oxide film is larger than the above thickness and the surface resistivity exceeds 5 mxcexa9xc2x7cm2, satisfactory electrical conductivity as the separator cannot be ensured.
In the invention, the thickness of the titanium or titanium alloy covering is preferably 0.1 to 2 mm, more preferably 0.1 to 0.5 mm, from the viewpoint of ensuring corrosion resistance on a level which is high enough to withstand use for a long period of time. The proportion of the covering of titanium or titanium alloy is preferably 5 to 20% in terms of volume ratio based on the whole cladding material.
Further, in the production process of a separator for a fuel cell according to the invention, in performing plastic working of the cladding material, after the covering of the surface of the material with copper, the plastic working is performed followed by the removal of the copper from the surface of the material. By virtue of this construction, the cladding material composed of a material having poor formability can be very easily formed.